Process for hydrodealkylating alkylsubstituted aromatic hydrocarbons



United States Patent 3,402,214 PROCESS FOR HYDRODEALKYLATING ALKYL-SUBSTITUTED AROMAT IC HYDROCARBONS Shinobu Masamun'e, Yasuo Suzuki,Yoshio Okabe, Mitsuo Sagara, and Hiroyuki Sagami, Yokkaichi, Mie, Japan,assignors to Mitsubishi Petrochemical Co., Ltd., Tokyo,

apan No Drawing. Filed Aug. 28, 1967, Ser. No. 663,518 Claims priority,applic/atisolasgapan, 'Sept. 8, 1966,

3 Claims. (Cl: 260-672) ABSTRACT OF THE DISCLOSURE Field of theinvention This invention relates to an improvement in the process forthe noncatalytic thermal hydrodealkylation of hydrocarbon oilscontaining alkyl-substituted aromatic hydrocarbons or hydrocarbon oilscontaining al'kyl-substituted aromatic hydrocarbons and nonaromatichydrocarbons.

For 'brevitys sake, alkyl-substituted aromatic hydrocarbons may bereferred to simply as alkyl aromatics hereinafter.

Description of the prior art Normally, the reaction in which aromatichydrocarbons are produced by the hydrodealkylation of alkyl aromatics ishighly exothermic. The amount of heat generated in the reaction variesconsiderably depending upon the type and number of alkyl groups presenton aromatic nucleus of the starting alkyl aromatics, and further uponthe relative configuration of alkyl groups, i.e., 0-, m-, ppositions,when two or more alkyl groups are present on the aromatic nucleus.

The heat of reaction in the hydrodealkylations of toluene and xylene,i.e., most conventional types of alkyl aromatics, are approximately 12Kcal./mol and 23 KcaL/mol, respectively.

When a fraction containing nonaromatic hydrocarbons such as paraffinic,naphthenic and olefinic hydrocarbons in addition to aromatichydrocarbons is used as a starting material, these nonaromatichydrocarbons are subjected to a thermal hydrogenolysis and they aredecomposed to light hydrocarbon gases having 1 to 2, sometimes 3, carbonatoms.

The heat of reaction in the thermal hydrodealkylation of thesenonaromatic hydrocarbons is several times as great as in thehydrodealkylation of alkyl aromatics.

In this instance, the amount of heat of reaction is dependent upon thenumber of carbon atoms and the molecular structures of these nonaromatichydrocarbons. For example, in the hydrodealkylation of nonaromatichydrocarbons of the general formula, C H wherein n represents numbers of6 to 8, there is generated a large amount of heat of reaction as greatas approximately 45-55 Kcal./mol.

Thus, one of the measures adopted in the prior art processes in order toremove such a great amount of heat of reaction so as to maintain thereaction system at a 3,402,214 Patented Sept. 17, 1968 'ice desiredlevel of reaction temperature has been to incorporate a material such asbenzene which is substantially inert in nature to the hydrodealkylationinto the starting alkyl aromatics or a hydrocarbon mixture containingthe same, preheating the reaction mixture up to a required temperature,and introducing the same subsequently to the reaction zone. In thisinstance, the extraneous benzene may be incorporated into the startingreactants from the beginning of the reaction as above, or, it ispossible that benzene formed in the reaction system by thehydrodeal-kylation of alkyl aromatics serves as a diluent as it is.

Another countermeasure to deal with the controlling of reactiontemperature has been the provision of side stream supplying ports atdifferent points along the reactionzone, by which a cooling agent isejected into the reaction zone so that the rapid temperature rising maybe prevented to maintain the temperature in the reaction zone at adesired level. In this method, starting hydrocarbon oil, liquidhydrocarbon products, hydrogen or hydrogen containing gas are normallyused as cooling agents.

Although these methods mentioned above are fairly successful in avoidingthe rapid rising of the reaction temperature, the economy of overallhydrodealkylation of alkyl aromatics is greatly degraded due to thecomplications in the equipment employed and difficulty in operation.

The first above-mentioned method in which a material inert in nature tothe hydrodealkylation reaction is incorporated into the reaction systemhas a great drawback in that it leads to the decrease in the partialpressure of alkyl aromatics to be hydrodealkylated, which in turn givesrise to the decrease in the reaction rate. As a result, the methodnecessitates larger capacities for the entire equipment including thereactor, preheater, gas-liquid separator, purifying unit, etc., for theproduction of a given amount of product, causing increase in the fixedcost per unit amount of the product, thus, the method is greatlyhandicapped from the economical point of vlew.

The second above-mentioned method in which the controlling oftemperature is accomplished by the injection of a cooling agent from theside stream supplying ports provided along the reaction zone also, as inthe first method, inevitably requires the enlargement of the equipmentstemmed from the use of surplus gas, i.e., cooling agent. Moreover,since the fluctuations in the temperature and the flowing amount of thecooling agent in this method lead to the change in the behavior ofreactants in the reaction zone, the operation in this method has beenextremely difiicult.

Summary of the invention It is, accordingly, an object of this inventionto provide a process for the noncatalytic thermal hydrodealkylation ofalkyl aromatics in a high efliciency free from inconveniencesaccompanied by the prior art processes.

Heretofore, the hydrodealkylation reaction has been normally conductedin an adiabatic piston flow-type reactor. In order to proceed thehydrodealkylation in such a reactor, it is absolutely necessary topreheat the starting gas up to a temperature at which thehydrodealkylation reaction is initiated, e.g., 580-650" C., at the inletof the reaction zone. Once the hydrodealkylation reaction is initiated,the reaction proceeds automatically thereafter with its own exothermicheat of reaction.

However, since the reaction rate is normally an exponential function ofthe temperature, the reaction temperature in the piston flow reactionzone rises rapidly with respect to the flowing direction of the reactionfluid, after the reaction is initiated.

The upper limit of the temperature rising in such adiabatic reactionzone is primarily determined by the amount of heat of reaction and thespecific heat of the reaction fluid. The value varies in a wide rangedepending upon the composition of the starting material used. For example, in the hydrodealkylation of toluene, it is approximately 120 C.,and, in the hydrodealkylation of a fraction containing nonarornatichydrocarbons, it can be as high as 150-300 C. Thus, if the temperatureof the starting gas is raised up to a temperature which is sufiicient toinitiate the hydrodealkylation reaction, e.g., 580- 650 C. or higher, atthe inlet of the reaction zone of the adiabatic piston flow-typereactor, then, the temperature at the outlet of the reaction zone isinevitably raised up to as high as -800-950 C.

On the other hand, the order of reaction in the hydrodalkylation is 1with respect to the partial pressure of alkyl aromatics and 0:5 withrespect to the partial pressure of hydrogen. Thus, the higher thereaction pressure, the more desirable from the standpoint of the orderof reaction, and in general, the operation is carried out under thepressure of from 1 to 60 kg./cm. (gauge), and most preferably from 10 to40 kg./cm. (gauge).

Accordingly, in the conventional hydrodealkylation reaction practisedheretofore, it is very likely that the reactor employed in the reactionis exposed to such a high pressure and a high temperature, i.e., SOD-950C., as described above. It is readily appreciated that the economicalprocurement of materials for equipment to withstand such hightemperature and pressure is by no means an easy matter.

Moreover, as the reaction temperature becomes higher, the formation ofby-produced dealkylation-condensation product is increased with resultin the lowering of selectivity. Thus, there have been proposed variousmethods of preventing the rapid rising of reaction temperature in thehydrodealkylation equipment heretofore. Nevertheless, these proposedmethods are accompanied by such inconveniences as described above.

We have found that the hydrodealkylation of alkyl aromatics can beconducted efliciently free from inconveniences accompanied by theconventional processes by conducting the hydrodealkylation reaction inat least two reaction zones, the first reaction zone comprising a backmixing-type reactor such as a hollow cylinder surrounded by a heatinsulating material, and a second or the subsequent reaction zone beinga conventional adiabatic piston flow-type reactor surrounded by a heatinsulating material and connected in series to said first reaction zone.

That is, in accordance with this invention, there is provided a processfor the noncatalytic thermal hydrodealkylation of alkyl aromatics whichcomprises preheating the alkyl aromatics or a hydrocarbon fractioncontaining the same and nonaromatic hydrocarbons together with from 2 to10 mols per mol of said starting oil, and passing the resultant mixturethrough two or more reaction zones, said first reaction zone being aback mixingtype reactor and said second or the subsequent reaction zonebeing adiabatic piston flow-type reactors.

The first reaction zone in the process of this invention is not anadiabatic piston flow-type reactor commonly used heretofore but anadiabatic back mixing-type reactor in which the reaction fluid iscompletely mixed in the reaction zone. By using this back mixing-typereactor, the temperature in the reaction zone may be stabilized at thedesired level without any rapid changes as in the piston flow-typereactor. Normally, in the adiabatic reaction system, the temperaturedifference between the inlet and outlet is constant, irrespective of theflowing condition of the reaction fluid in the reaction zone, i.e., .thedifference is invariably constant regardless of the type of reactionzone. However, there is a remarkable difference between the pistonflow-type reactor and the back mixingtyp'e reactor in that the formeressentially requires high 4 temperature at the inlet of the reactionzone which is sufficient to initiate the hydrodealkylation reaction,whereas in the case of the latter no such arrangement is required. Thereason is that in the back mixing-type reactor the gaseous reactantscontained therein are completely mixed together and the temperature ismaintained at a certain level. Thus, in the back mixing-type reactor, itis possible to regulate the temperature in the reaction zone so as'notto unnecessarily exceed the temperature at which the reaction isinitiated. In the'back mixing-type reactor, since the temperature in thereaction zone is the same as that at the outlet thereof, the'startingmaterial may be supplied to the inlet of the reaction zone at atemperature which is lower by the difference between the temperatures atinlet and outlet of reaction zone which is determined by the heat ofreaction and the specific heat of the reaction fluid. Therefore, forexample, assuming that a temperature of the reaction zone at which thereaction is initiated is 650 C., the temperature at the inlet of thereaction zone can be lowered to the order of 400500 C. In other words,by using the back mixingtype reactor, the reaction can be conducted atlower temperature, e.g., lower by 200-300 C., as compared with the useof the piston flow-type reactor, and furthermore, the temperature at theinlet of the reaction zone can also be lowered to the same degree.Consequently, the reaction can be operated at an optional temperaturewithin the range including a lower limit of 580 C. and an upper limit of850 C. which is restricted from the standpoints of material for theequipment and the occurrence of undesirable side reactions.

The point mentioned above not only enables to reduce the burden imposedon the preheater in the reaction which calls for a heavy pressure and ahigh reaction temperature but also to suppress the reaction in thepreheater to the lowest minimum and facilitates the easier control oftemperature at the inlet of the reaction zone. In addition, there arebrought about a number of advantages from the commercial points of Viewin that the equipment can be made less expensively; that the operationcan be made easier due to the leveling of the temperature in thereaction zone; and that the selectivity of the reaction can be enhanceddue to the suppression of formation of byproduceddealkylation-condensation products, as a result of proceeding of thereaction in the low temperature region. Furthermore, the adiabatic backmixing-type reactor has an advantage in that the capacity of the reactorcan be made smaller, e.g. /2-Vs, as compared with that of the pistonflow-type reactor, up to a certain rate of conversion.

However, once the reaction is proceeded up to a certain degree, thereaction becomes nearly an isothermal reaction even in the piston flowreaction, and the back mixing-type reactor requires greatly prolongedresidence time to obtain a high conversion beyond the certain level.Thus, if the reaction were to be carried out up to the desired rate ofconversion in the back mixing-type reaction zone alone, the size of thereactor should be very large.

In addition, when nonaromatic hydrocarbons are contained in the startingoil, some of them are short circuited because of the wider distributionof residence times and they are discharged unreacted without beingdecomposed to light hydrocarbon gases having 1-3 carbon atoms. Theseunreacted nonaromatic hydrocarbons give an azeotropic mixture withbenzene produced in the hydrodealkylation, which maynot be separated bydistillations.

The remaining of such azeotropic mixture in the final product, ofcourse, leads to the degraded purity of the product benzene which givesrise to lowering of the freezing point, widening of the distillationrange, deterioration in the sulfuric acid coloring test and so forth.

In order to overcome this problem, it is necessary that thosenonaromatic hydrocarbons are converted completely in the reaction zoneto light hydrocarbons which can be easily separated from the liquidproduct by a liquid-gas separator.

We have found that nonaromatic hydrocarbons are converted completely tolight hydrocarbons by arranging the second or subsequent adiabatic'piston flow-type reaction zones connected in series with the backmixingtype reaction zone of a first reaction stage. In the adiabaticpiston flow-type reaction zone, the distribution of residence time isnarrowed and n0 short circuiting of reactants occurs, in other words,there is provided an equal residence time for any part of any reactionfluid. Thus, as the reaction is proceeded up to a certain extent and theconcentration of reactants in the reaction fluid is decreased, the backmixing-type reaction zone requires quite large capacity for the reactorin order to carry on the reaction beyond that point, whereas in thepiston flowtype reaction zone where no short circuiting takes place cando with the smaller capacity for the reactor as compared with the backmixing-type reaction zone.

From such point of view, in the process of this invention there isprovided a second and the subsequent stages of the piston flow-typereaction zones and, by so arranging, when the unreacted alkyl aromaticsdischarged from the first stage are hydrodealkylated up to the desiredrate of conversion, nonaromatic hydrocarbons contained in the startingoil are converted completely to light hydrocarbon gases. Thus, there isbrought about a remarkable advantage by the combination of the backmixing-type reaction zone of the first stage and the piston flow-typereaction zones of the second and the subsequent stages.

In carrying out the back mixing in the first stage of the process ofthis invention, the provision of a stirrer to the reactor is desirableas in the conventional liquidphase reactor. However, the stirring ofhigh temperature gas under such a high pressure as in the instantreaction, e.g., 1-60 kg/cm? (gauge), and preferably 1040 kg./ cm.(gauge), is not an easy matter due to the mechanical difficulty involvedin the insertion of the stirrer intothe reactor. Therefore, in theprocess of this invention, it is suitable to adopt a jet-type internalrecycling reactor utilizing kinetic energy of moving gas or a way ofinternal mixing by feeding a fluid from the tangent direction of thecylinder along the inside wall thereof.

The reactors which may be used in the process of this invention in thesecond and the subsequent stages can be of the conventional adiabatichollow cylinder-type condition. If necessary, hydrogen as a coolingagent may be fed to the reaction mixture between the first and thesubsequent stage reactors.

The starting materials which may be used in the process of thisinvention are alkyl aromatics or hydrocarbon fraction containing thesame. They include alkyl aromatics such as toluene, xylene,ethylbenzene, propylbenzene, methylethylbenzene, trimethylbenzene anddiethylbenzene, and hydrocarbon fractions containing nonaromatichydrocarbons such as paraflinic, naphthenic and olefinic hydrocarbons.

Hydrogen gas containing impurities such as light hydrocarbon gases suchas methane, ethane and propane, carbon monoxide and carbon dioxide maybe used in the process of this invention without giving any adverseeffect so long as the purity is more than 40% by volume, and preferablymore than 60% by volume.

The amount of hydrogen to be fed to the reaction zone is more than onemol, and preferably from 2 to mols, per mol of starting oil. The use ofless than 2 mols of hydrogen per mol of the starting oil leads to thedecrease in the rate of dealkylationand increases the formation of tarrymaterial which is lay-produced in the hydrodealkylation reaction thusdecreasing the yield of aromatic hydrocarbons. In addition, it sometimesgives rise to the clogging of reactor due to the formation of carbon.However, the use of excessive amount of hydrogen, e.g., more than 10mols per mol of the starting oil gives no particular increase in theeffect thus it is disadvantageous from the economical point of view.

The temperatures in the reaction zones in the process of this inventionare preferably from 580 C. at which the reaction is initiated to 800 C.,and the pressure is l-60 kg./cm. (gauge), and preferably 10-40 kg./cm.(gauge) from the economical standpoint, though the higher the pressure,the more advantageous theoretically as described above.

In summary, by the provision of two or more separated reaction zones inthe hydrodealkylation process according to this invention, there arebrought about many advantages mentioned below all of which contribute tothe economical hydrodealkylation of hydrocarbons on a commercial scale,namely:

(1) The control of temperature in the highly exothermic reaction can bemade easier,

(2) Starting material can be .fed to the reaction zone at a lowtemperature,

(3) Recycling of product for the controlling of reaction temperature isnot necessary,

(4) No jetting of hydrogen or inert gas as cooling agent for the controlof temperature in the reaction zone is required,

(5 Deal-kylated product of high purity can be obtained, and

(6) Reactor of a smaller capacity may be conveniently used.

Description of preferred embodiments This invention may be explainedmore fully in the following examples. However, it should not beconstrued that the examples restrict this invention as they are givenmerely by way of illustration:

EXAMPLE 1 Cracked residue by-produced in the steam cracking of naphthain the production of ethylene was hydrogenated and the product was usedas a starting material. The material had a boiling range ofapproximately -180 C. and the composition was as shown below:

Aromatic hydrocarbons: Percent by weight Benzene 25.5 Toluene 21.0Xylene 15.7 Nonaromatic hydrocarbons 37.8

Total 100.0

After the starting oil mentioned above was mixed with 6 mols of hydrogenpressurized at about 23 atmospheres per mol of said starting oil, andheated in a preheater up to 510 C., the resulting mixture was introducedto a first stage, i.e., a back mixing-type reaction zone andsubsequently to a second stage, i.e., piston flow-type reaction Zone.When of alkyl aromatics were converted, a temperature at an outlet ofthe first reaction zone was 670 C. and that of the second reaction zonewas 680 C. The composition of the resulting product was shown in Table 1shown hereinafter.

COMPARATIVE EXAMPLE 1 COMPARATIVE EXAMPLE 2 The same material andhydrogen as used in Example 1 were mixed and heated according to thesame procedures as described therein and the resulting mixture was fedto TABLE 1.COMPOSITION OF PRODUCTS Products Example 1 Comparative 1Comparative 2 Benzene 47. 3 42. 47. 2 Toluene 5.2 5. 2 5. 3 Xylene 1.01.1 1.0 N oil-aromatics 5. O Tarry matter 3. 3 8. 2 3. 2

1 Percent by weight based on the weight of the starting oil.

As can be noted from the above, in Comparative Example 1, the reactiontemperature was raised up to 800 C. and the formation of tarry matterwas increased, which in turn, decreased the yield of benzene.

In Comparative Example 2, although the formation of tarry matter wasdecreased due to the lower reaction temperature, there is a drawback inthat it gives more nonaromatics which have not been decomposed.

In the process of this invention, on the other hand, the formation oftarry matter is decreased due to the lower reaction temperature andnonaromatics are decomposed completely and ceased to exist.

EXAMPLE 2 Required Relative Type of reactor capacity ratio of (1113*)required capacity Adiabatic piston flow-type 1. 45 1. 92 Adiabatic backmixing-type- 1. 08 1. 43 Combination of adiabatic back mixing-type andadiabatic piston flow-type 0. 755 1. 00

As can be noted from the'above, the prior processes require largercapacity for the reactor by 1.4-1.9 times as compared with the processof this invention.

We claim:

1. A process for the noncatalytic thermal hydrodealkylation ofalkyl-substituted aromatic hydrocarbons which comprises preheatinghydrocarbon oils containing a member selected from the group consistingof alkyl-substituted aromatic hydrocarbon and a mixture consisting ofalkylsubstituted aromatic hydrocarbon and nonaromatic hydrocarbon,together with hydrogen, and passing the preheated mixture through atleast two independent reactors arranged in series and maintained athydrodealkylating conditions, the first of said reactors being a backmixing-type reactor to effect substantially complete mixing of thecomponents 01'. the preheated mixture with the reactants andstabilization of the temperature in said first reactor, and eachsubsequent reactor being a piston flow-type unpacked reactor.

2. A process according to claim 1 wherein said hydrogen is used in anamount of 2 to 10 mols per mol of the starting alkyl-substitutedaromatic hydrocarbons, the temperatures of the reaction zones are from580 to 800 C., and the pressure is from 1 to kg./cm. (gauge).

3. A process according to claim 1, further comprising contacting thereaction mixture passing from the first reactor to the next reactor withan additional source of hydrogen to cool the mixture.

References Cited UNITED STATES PATENTS 3,188,359 6/1965 Lempert et al260672 3,198,847 8/1965 Lanning 260672 3,284,525 11/1966 Begley 2606723,287,431 11/ 1966 Feigelman 260672 3,288,873 11/1966 Moll 2606723,288,874 11/1966 Bowles 2604672 3,288,876 11/1966 Hammond et al. 260672OTHER REFERENCES Fowle & Pitts: Thermal Hydrodealkylation, CEP 58(4),37-40 (1962).

Feigelman et al: Thermally Dealkylate Toluene, HP 44(12), 147-150(1965).

DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMITKONS, Assistant Examiner.

